1. Field of the Invention
The present invention relates to internal combustion engines. More particularly, the invention relates to methods and apparatus for reducing the manifold conduit pressure losses by fuel vapor flows to combustion chamber and exhaust gas flows therefrom.
2. Description of the Prior Art
Motor vehicle designers seek the smallest possible power plant so that more of the vehicle volume can be used for passengers and cargo. Equipping a vehicle with a smaller power plant reduces the weight leading to additional fuel efficiency. Engine designers seek engines that convert the heat energy in fuel to shaft power as efficiently as possible and, simultaneously, to get as much air and fuel into and out of the engine as possible. These two design goals result in high output for a given engine size. Unfortunately, an engine that meets these design goals will not be of the smallest possible volume, because of the fluid mechanics of fluid flow.
When a flow stream moves around a corner, the molecules in the fluid seek the path of least resistance-the molecules go to low pressure. Pressure gradients are caused by variations in the speed of the flow stream and by friction. High speed portions of the flow stream are at low pressures while low speed portions are at high pressure.
In a turn, the highest speeds and lowest pressures are found in the inside radius of the turn at the apex, or midpoint of the turn. The highest pressures (and lowest velocities) are found just before the start of the turn on the inside, on the outside apex of the turn, and on the inside just after the turn. Molecules approaching the turn on the side of the duct near the inside of the turn slow. They are caught up in the space upstream of the high velocity molecules passing near the inside around the apex of the turn. Thus, at the beginning of the turn most of the flow occurs near the wall opposite the inside of the turn or on the outside of the turn. When the flow reaches the apex of the turn, most of the flow is at the inside of the duct going at high speeds. This momentum carries the flow to the outside of the duct at the end of the turn, forming three spaces of stagnant or low flow. These spaces are: upstream of the turn on the inside wall, in the middle of the turn on the outside, and on the inside wall on the downstream side of the turn.
This flow pattern is readily seen in a river. Just before a bend in the river the flow is on the bank opposite the turn, Half way through the bend, the flow is against the bank on the inside of the bend. The main part of the flow then moves back to the outside bank downstream of the bend. This flow pattern is inefficient. It does not use all the area of the duct and requires energy to be used to speed up and slow portions of the total flow. In addition, some slow-moving flow in the three stagnant areas is swept into the main flow stream causing momentum losses. Another way to consider this is that, energy in the flow stream must be used to force the flow around sharp bends. If the river turns less than nine degrees this effect is not seen--the flow stays constant across the area and there is little pressure drop and flow loss.
The problem of flow loss caused by changing the direction of a flow stream has been studied in fluid mechanics for many years. It has been found that the pressure drop caused by a flow direction change is equal to the following: ##EQU1## Where P.sub.d is the pressure drop across the turn, K is the loss coefficient, V is the average velocity of the flow stream, and D is the density of the fluid that is flowing.
Finding the loss coefficient is difficult; experimentation is the usual method. However, when the geometry is simple, super computers can model the loss coefficient accurately. Fluid mechanics texts, handbooks, and other references have tables, graphs, and charts that relate the loss coefficient to the various dimensions of turning flow streams. These references offer little on how to reduce the loss coefficient other than by increasing the inside radius of the flow stream. Close inspection of this data does show that flow streams in rectangular ducts have smaller loss coefficients than flow streams in ducts of circular cross-section.
Also, from this data, a bend in the flow stream with the smallest loss coefficient has an inside radius that is twice the diameter of the duct. Ducts with turn radius to diameter ratios larger than two have increasingly larger loss coefficients due to increasing wall friction caused by the longer turn length. The inlet port in current V-8 engines is approximately three inches high and makes nearly a ninety degree turn. A low flow loss port would have a six inch inside radius. Add to that the three inch port and some metal at each end and the cylinder head would be over ten inches tall. Such an engine would also require long stem valves and stronger, mechanisms to operate the longer and heavier valves.
Engines that move large quantities of air and fuel also have efficient valve mechanisms that control the flow into and out of the engine with the least amount of flow resistance. High flow passages also use space that other engine structures such as head bolts, cooling passages, oiling passages, and valve actuation mechanisms must have. Consequently, the engine must be even larger to hold these components. Clearly, such an engine is impractical.
Naturally aspirated engines--engines that are not pressurized by turbochargers or superchargers--fill their cylinders from a constant pressure source. It is the atmosphere. The difference between atmospheric pressure and the pressure in the cylinder during the intake stroke provides the differential to fill the cylinder. This differential must accelerate the incoming flow stream and overcome flow losses.
Ideally, the cylinder attains full atmospheric pressure when it is filled with air and fuel. This gets the maximum mass of air and fuel into the cylinder for maximum output. Flow losses in the air intake system, air cleaner, carburetor, intake manifold, cylinder head, and around the intake valve prevent this. Since air is compressible, the cylinder does not get filled with air and fuel at atmospheric pressure. The density of the charge is lower because the pressure is lower in the cylinder due to flow losses. Less air and fuel are available for combustion, resulting in less than full power from each cylinder and less than full power from the engine.
The flow loss pressure differential is found, using the equation given above. Thus, the flow loss pressure differential of the intake air fuel mixture increases with the square of the velocity of the intake air fuel mixture. At higher engine revolutions the total pressure drop from flow losses increases rapidly; engine power drops dramatically.
Once engine speeds, and inlet flow stream velocities, near the point where the loss coefficient pressure nearly equals the pressure drop created by the engine's moving pistons, engine power decreases by the cube with additional increases in engine speed. This is because the air fuel mixture is compressible; The mass of air getting into the engine is a linear function of cylinder pressure that is decreasing by the square of the velocity. The amount of mass entering the cylinder drops by the cube of engine speed. Small reductions in the loss coefficient result in large improvements in peak power production.
Designers have achieved the required engine power output by operating their engines at lower shaft speeds and increasing engine displacement to overcome the effects of the low flow inlet and outlet passages of current design. This keeps the engine smaller than one with ideal flow passages, however, the engine displacement must be larger to produce the same power. The engine is heavier adding unnecessarily to vehicle weight. Even more fuel is needed to move the heavier vehicle.
One approach designers have used is to provide parallel paths into and out of the engine. The result is designs with four valves per cylinder that have higher turn radius to diameter ratios and better use the space in the combustion chamber for flow. This approach adds complexity and increases the size of the design. Also, the smaller passages result in higher wall friction losses. All the complexity adds to the cost of manufacture, raising the price of the motor vehicle.
This invention will show a way to increase the flow in and out of an engine. The invention may be used in new engine designs or retrofitted to existing designs already in service.